net: sched: taprio: fix -Wmissing-prototypes warnings

We get one warnings when build kernel W=1:
net/sched/sch_taprio.c:1155:6: warning: no previous prototype for ‘taprio_offload_config_changed’ [-Wmissing-prototypes]

Make the function static to fix this.

Fixes: 9c66d156 ("taprio: Add support for hardware offloading")
Signed-off-by: Yi Wang <wang.yi59@zte.com.cn>
Acked-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>